Such drill lances are used with apparatus which serve to drill underground channels. Fluid under pressure exits from the forwardly directed nozzle, for example a water-bentonite-milk. An even drilling in the forward direction through substantially homogeneous earth is achieved by rotating the drill head together with the drilling lance with an even rotational velocity about the center axis. When changes in direction are necessary, the rotational movement is stopped as soon as the nozzle of the drill head is located approximately in the position at which the change in direction is to begin. Earth is preferably only removed in one direction through the fluid jet emitting out of the nozzle. In accordance with the effected removal, the entire boring lance is turned until the new direction has been set. This hydromechanical steering is often supported by a control surface which is formed by a chamferring of the drill head located opposite the nozzle. The rotational movement of the drill head is then started again and a linear drilling ensues in the new direction.
Such a drill head for a drill lance is known from EP-0 195 559 A1. In order to take the various conditions in the earth into account, in particular its hardness, a different number of nozzles is selected accordingly. With this drill lance, sufficiently exact drillings can be carried out so long as the underground consists of relatively soft earth. In the case of hard earth material, difficulties arise in the stabilisation of the drill lance, as the side of the drilling lance lying opposite the nozzle arrangement hits against the hard rock which results in stearing problems with the lance during the relatively slow advance.
In order to be able to work hard earth all the same, it is suggested according to U.S. Pat. No. 2,324,102 to use chemicals or acids as high pressure fluid which attack the rock. As soon as the applied chemical has loosened the rock, the drill lance follows through as it only has to force its way through relatively soft material. This solution is often not desirable, as this results in impairment of the environment or the ground water.
It is an object of the invention to provide a drill lance with which directionally accurate drillings can be carried out in compact, sandy and gravelly ground as well as in hard ground. This object is solved by a drill lance of a type initially mentioned having the features of the characterizing clause of claim 1. Advantageous embodiments are outlined in the dependent claims.